Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 405

2

U f
U c

d c = d f

(6-13)

This is an approximation that converges properly for uniform flow with dissipation as well
as for non-uniform flow without dissipation, and thus it may be expected to be a reasonable
model for the case of non-uniform flow with dissipation.

Measurement of Wake Effects

Wind Tunnel Wake Tests

Wind tunnel tests have been conducted to measure wake deficits behind single turbines.
These have established the strong dependence of the wake deficit on the tunnel turbulence,
as predicted by the theory expressed in Equations (6-9). They have also shown reasonable
correlation with the semi-empirical model in decay law (the functional dependence of deficit
with downwind distance) and the actual rate of the decay (the magnitude of the decay
constant). A typical example of measured deficits compared with model predictions is
shown in Figure 6-18. Both the data and the model indicate that the centerline velocity
deficit ( U - V min ) decays at a rate inversely proportional to the downwind distance x.
Turbulence intensities in wind tunnels are generally low, even when they are artificially
increased by the use of wire grids and boundary layer devices, as was done during these
tests. However, it can be seen that even for these low turbulences an increase in the
turbulence intensity of only 0.01 significantly increases the wake decay rate.
Because of the difficulty of reproducing the actual nature and scales of the ambient
turbulence and other full-scale factors relating to the turbine rotor, the prospects of
validating an analytical wake model in a wind tunnel to a higher degree than that shown
in Figure 6-18 are quite low.